1. Field of the Invention
Embodiments of the invention relate to a low temperature cofired ceramics (LTCC) substrate structure with at least one contact element for connecting a wire conductor, which has a first metallization arranged on and/or in the ceramic substrate, which metallization preferably contains silver or a silver alloy, for electrical connection to the wire conductor, and a method for producing a substrate structure of this type.
2. Description of the Related Art
Currently, substrate structures for ceramic printed circuit boards are often produced by means of the so-called LTCC technology (LTCC=low temperature cofired ceramics). This technology renders possible a cost-effective production of multilayer circuits with integrated passive elements on the basis of sintered ceramic substrates. The term “cofired” hereby describes the possibility of firing conductor paths jointly with the ceramic in the LTCC process.
To connect the circuit structures by means of wire conductors (lead wires), contact elements in the form of bonding pads or bonding islands are often provided on the ceramic substrate, which comprising an island are composed of a first metal layer arranged on the ceramic substrate. The wire conductors are used to electrically connect a circuit structure to further circuit structures, for example, a semiconductor chip. Electrically conductive materials such as gold, a gold alloy, copper or a copper alloy are therefore used as wire conductors. To produce the connection, the contact element is connected to the lead wire, for example, by means of soldering or other connecting methods (bonding).
Bonding (to be more exact: wire bonding) is used in electrical engineering to describe a joining technique in which a connection is produced between electronic elements of an electric circuit with the aid of a thin wire (wire connector, lead wire) by welding the wire with the land. The welding can be carried out, for example, by means of thermal activation (thermosonic bonding) or by means of ultrasound (ultrasonic bonding), the advantage of the ultrasound-supported wire bonding being that a bond connection can be generated at room temperature, wherein special prerequisites have to be met with gold wires for room temperature bonding. One of the prerequisites is, for example, the coating of the gold wires with aluminum. The aluminum, which deposits in islands, oxidizes to aluminum oxide. In the bonding process this coating serves as an abrasive medium that promotes the room temperature bonding process through an abrasive effect.
Soldering is a thermal method for joining materials by adhesive force, wherein a liquid phase is produced by melting a solder (melt soldering). The solidus temperature of the base materials is not reached during soldering. The solder material is an easily fusible metal alloy, with the aid of which a metallic connection of two metallic components is produced. The solder with the materials to be joined forms one or more intermetallic phases at the joint, which produces a firm connection of the materials to be joined to one another.
In order to produce a connection between the first metallization (also referred to below as metal layer) and the wire conductor, two different metallization types are currently used with the LTCC technology. The first variant is a pure gold system, wherein pure gold pastes supplemented with mixtures capable of being soldered (AuPtPd and the like) are used for printing. However, this is very cost-intensive. The second system is referred to as a so-called mixed metal system, which contains a number of agentiferous paste mixtures in addition to pure silver pastes, depending on the use.
If the gold pad and the silver-containing metallization lie one on top of the other, the problem arises that the noble metals gold and silver, which have different diffusion rates, come into direct contact, so that with the diffusion of both noble metals into one another a volume change occurs that causes a cavity formation (so-called Kirkendall voiding). The cavity formation can lead to a defective nature of the transition and thus to an interruption of the electrical connection, which cannot be tolerated in all of the substrates used in particular in medical technology.
A transition of this type has therefore hitherto usually been realized through so-called transition vias (through-hole plating) of an AgPd alloy, which, however, tend to a mushroom like growth out of the tape due to a mismatching to the tape through the sintering.
One solution of the problem described above would be the application of a diffusion barrier layer between the gold pad and the silver metallization by means of a plating method acting globally over the entire ceramic substrate on platable silver as the first metal layer. Plating is a currentless deposition of one material on another. However, a plating process of this type requires a large investment of time in the development of the process for the concrete application. Furthermore, the process is very error-prone. The substrate structures—and hereby in particular the connections—must operate without errors, in particular for medical applications. Moreover, there is the problem with plating that the baths used for the plating have to be loaded continuously in order to avoid a lasting change in their composition (transition). For smaller enterprises a production line that contains a plating step is therefore very expensive. The integral plating process is therefore not suitable for the application of the diffusion barrier layer in the case of small throughputs.
A method for an ink-jet printing of a contact element capable of being soldered on a substrate for the production of PCBs (printed circuit boards) is known from WO 2007/140463 A2. With PCBs, the mask and other structures are printed onto the finished substrate. In order to produce a contact element, with the known methods first of all a first layer is applied to a substrate with a first ink, wherein the first ink has a relatively high conductivity. This first layer is subsequently subjected to a thermal treatment. Subsequently an intermediate layer with a low conductivity is applied and subsequently likewise subjected to a thermal treatment. Finally, a second ink is applied to the intermediate layer, which ink is subsequently likewise thermally treated. The third layer serves to produce the soldered joint.